1. Field of the Invention
The present invention relates to a mold for bending a glass sheet into a predetermined shape.
2. Discussion of the Background
Glass sheets having a configuration corresponding to that of a curved portion or a corner of an automobile and a building have been widely used in the automobile industry, the construction industry and so on to thereby seek comfort by widening view and increasing lighting efficiency, or to obtain a beautiful appearance. The glass sheet having a curved or bent portion is manufactured, for instance, by heating the glass sheet at a temperature suitable for bending it, pressing the heated glass sheet by a metal mold having a predetermined shape, and then blowing air to the glass sheet to rapidly cool it while keeping a shaped configuration, or leaving the glass sheet suspended in air until the heated glass sheet approaches a predetermined temperature. For this purpose, various methods and apparatuses for conducting the methods have been proposed. However, in any method and apparatus, it is necessary to use a metal mold to shape the glass sheet into a predetermined configuration. The metal mold is usually fubricated by processing a plate or plates made of stainless steel.
For instance, when a curved glass sheet having a complicated shape such as a glass sheet having a three-dimensionally curved surface used for rear windows, front windows, side windows, roof windows for automobiles is to be manufactured, a metal mold should be so designed and shaped as to have a curved surface by which a final glass sheet product having a curved surface corresponding to that of the metal mold is formed. Heretofore, the metal mold is prepared as follows. A stainless steel plate is divided into a predetermined number of cut pieces; each of the cut pieces is bend-shaped to have a predetermined shape; the cut pieces are successively joined by welding so as to form a curved surface having a three-dimensional configuration; reinforcing ribs are attached on the back surface of the joined plates by welding; and a front surface portion to which a heated glass sheet is brought into contact (hereinbelow, referred to as a glass sheet contacting surface) is precisely shaved or ground by a numerically-controlled shaving machine or grinding machine to obtain a predetermined shape contacting surface.
However, in the conventional mold, the various steps of preparation of the cut-pieces, welding of the cut pieces, welding of the reinforcing ribs on the back surface of the mold and so on are required. Accordingly, the wall thickness of a stainless steel plate usable as a material for the mold is limited from the standpoint of ease in bending of each of the cut pieces and welding them. Usually, it is difficult to use the stainless steel plate having a thickness of about 6 mm or more. Actually, a stainless steel plate having a thickness of less than 6 mm is used because the glass sheet contacting surface of the mold has to be processed for fine finishing by an NC shaving or grinding method. In addition, use of the stainless steel plate primarily determines the wall thickness, whereby flexibility in attaching the reinforcing ribs is greatly limited, and flexibility in distribution of the wall thickness is also greatly limited.
In the conventional metal mold, there may be a case where a metal mold having a finely finished glass sheet contacting surface as expected can not be obtained. There may be a case where that a metal mold having a predetermined configuration of glass sheet contacting surface is used for a long time and the glass sheet contacting surface of the mold has to be adjusted again. Or, there may be a case where it is necessary to change a curved portion in the glass sheet contacting surface in the course of a trial manufacturing stage of a curved glass sheet to a final product manufacturing stage. In these cases, it is difficult to obtain a complete mold by only the NC shaving or grinding method, and there may be a case where a fresh mold has to be prepared even by a slight change of the glass sheet which is finally produced.
Further, the metal mold fabricated by bending each of the cut pieces of the stainless steel plate followed by welding them easily suffers from thermal deformation when the mold is used under a high temperature condition.